Method for establishing a secure communication channel

ABSTRACT

In a method for establishing a secure communication channel between a portable data carrier ( 10 ) and a terminal on the basis of an asymmetric cryptosystem, a value (X; Y; V; W) derived from a public key (PKD; PKT) of the cryptosystem is displayed on a display device ( 40 ) of the data carrier ( 10 ).

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to a method for setting up a securedcommunication channel between a portable data carrier and a terminal onthe basis of an asymmetric crypto system as well as to a correspondinglyadapted portable data carrier.

B. Related Art

A secured data communication between a portable data carrier, forexample a chip card, and a terminal adapted for the communication withthe data carrier is effected via a secured communication channel, by thetransmitted data being transmitted in an encrypted manner. The datatransmission here can be effected in a contact-type or contactlessmanner. An efficient and secure encryption is ensured by means of knownsymmetric encryption techniques, for example AES, DES or the like.However, for this, the exchange of a secret key between the two partnersis necessary.

This exchange is normally effected by means of an asymmetric method, forexample the Diffie-Hellman key exchange method. Such an asymmetricmethod, with which public keys of a corresponding crypto system aretransmitted via an insecure communication channel, is vulnerable toso-called man-in-the-middle attacks (MIM). Here, an attacker eavesdropson the unsecured communication channel and pretends vis-à-vis each ofthe two communication partners to be the respective other one, withoutthis being recognizable to the two communication partners. In this waythe attacker can eavesdrop on the entire data communication and alsomanipulate it, in particular can obtain an exchanged “secret” key andwith this manipulate the following data communication between the datacarrier and the terminal.

It is the object of the present invention to support the setting up of asecured communication channel between a portable data carrier and aterminal, in particular to make man-in-the-middle attacks against thecommunication channel more difficult.

SUMMARY OF THE INVENTION

The invention is based on the basic idea to include a display device ofthe data carrier in the method, in particular for authenticating apublic key of the crypto system.

Accordingly, upon a method according to the invention for setting up asecured communication channel between a portable data carrier and aterminal on the basis of an asymmetric crypto system, a value derivedfrom a public key of the crypto system is displayed on a display deviceof the data carrier.

A corresponding data carrier according to the invention comprises adisplay device as well as a control device for setting up a securedcommunication channel to a terminal on the basis of an asymmetric cryptosystem via a data communication interface of the data carrier. Thecontrol device here is adapted, according to the invention, to display avalue derived from a public key of the crypto system on the displaydevice. Preferably, the data carrier has an input device.

In this way, it is possible to prevent a man-in-the-middle attack on thecommunication channel or at least to recognize it. Upon such an attack,the attacker intercepts a public key transmitted via the communicationchannel and replaces it by a different key chosen by the attackerhimself When, however, on the display device of the data carrier thereis displayed a value, which for example has been derived from the publickey—supposedly—received from the terminal, e.g. a so-called fingerprintof the key, a user of the data carrier can visually check whether thedisplayed value matches the publicly known fingerprint of the public keyof the terminal. If this is not the case, then the key received by thedata carrier does not correspond to the public key of the terminal, butto a key smuggled in by an attacker. An attack on the communicationchannel can therefore be reliably recognized and an unsecuredtransmission of sensitive data be effectively prevented.

Preferably, also on a display device of the terminal there is displayeda corresponding value derived from a public key, for example theabove-mentioned fingerprint. This facilitates the direct visual check ofthe identity of the derived value displayed on the display device of thedata carrier with the corresponding value that is known but additionallydisplayed on the display device of the terminal.

According to a preferred embodiment of the method according to theinvention, there are displayed both on the display device of the datacarrier and on the display device of the terminal, preferably one afterthe other, both a value derived from a public key of the data carrierand a value derived from a public key assigned to the terminal. Thisembodiment is particularly suitable for securing the transmission of thepublic keys between the data carrier and the terminal during aDiffie-Hellman key exchange method. Here, one communication partnersends a public key assigned to him to the respective other communicationpartner. Through the both-sided displaying of the values derived fromthe sent and received keys, a user of the data carrier can visuallycheck whether the public key sent by his data carrier to the terminalhas been received there unchanged, on the one hand, and whether the keyreceived by the data carrier actually corresponds to the public key ofthe terminal, on the other hand.

According to a further variant of the invention, at the beginning of thecommunication between terminal and data carrier there is respectivelyexchanged a derived value of the public key assigned to thecommunication partner. In a following method step, then the respectivepublic key is transmitted and its derived value verified by thecommunication partner. Subsequently, the terminal forms, according to anasymmetric encryption based on a Diffie-Hellman key exchange method, acommon secret from the public key of the data carrier and the privatekey of the terminal, while the data carrier forms the common secret fromthe public key of the terminal and the private key of the data carrier.The derived value of the common secret is transmitted to the respectiveother communication partner. In this variant, too, a user of the datacarrier can visually check whether the common secret sent by his datacarrier to the terminal has been received there unchanged, on the onehand, and whether the secret received by the data carrier actuallycorresponds to the sent secret of the terminal, on the other hand.

According to a further variant of the inventive method, the data carriergenerates a random number and transmits a derived value of the randomnumber and a public key assigned to the data carrier to the terminal.Further, by the terminal there is generated a random number, this istransmitted, encrypted with the public key of the data carrier, to thedata carrier and subsequently decrypted by it. Subsequently, the randomnumber generated by the data carrier is transmitted to the terminal andthe derived value thereof is verified by the terminal. In a furtherstep, both the terminal and the data carrier form a derived value fromthe random number of the card, from the random number of the terminal,and from the public key of the data carrier and display it. Here, too,the user again can visually check the match of the values.

In other cryptographic methods for setting up a secured communicationchannel, upon which only one public key is transmitted from onecommunication partner to the other, there is accordingly omitted thedisplay of the other—not sent and not received—public key of the othercommunication partner.

The visual check by the user of the data carrier as to whether a valuedisplayed on the display device of the data carrier matches acorresponding value displayed on the display device of the terminal,whereby the two values have been respectively derived from a same publickey in the same way, can be replaced by an automatic check in the datacarrier or in the terminal. If the data carrier comprises an inputdevice, for example a keyboard, then the value displayed on the displaydevice of the terminal, which has been derived for example from thepublic key of the terminal, can be input to the data carrier via thekeyboard. The control device of the data carrier is then adapted tocompare the input value with a value which has been derived in aspecified manner from the public key of the terminal received by thedata carrier from the terminal. Non-matching values indicate an attackon the communication channel.

In an analogous way, also a value displayed on the display device of thedata carrier, which for example has been derived from the public key ofthe data carrier, can be input to the terminal via an input device ofthe terminal and be compared there in the described manner with a valuewhich has been derived from a public key of the data carrier received bythe terminal from the data carrier.

Alternatively or additionally, the other derived values can be displayedand be input to the other apparatus for checking, i.e. the valuedisplayed on the display device of the terminal, which has been derivedfrom the data carrier's public key received by the terminal, can beinput to the data carrier for checking, and/or the value displayed onthe display device of the data carrier, which has been derived from theterminal's public key received by the data carrier, can be input to theterminal for checking.

A value derived from a public key of the crypto system, such as forexample a fingerprint of a key, is preferably formed by means of a hashfunction. Here, the entire hash value or optionally only a part of itmay be displayed, for example when a display device of the data carriercan only display a limited number of digits of such a value. It is alsopossible to form the hash value or the like not directly from the publickey, but from a value which in turn depends on the public key, forexample a secret key formed by means of the public key, which secret keyis only known to the data carrier and the terminal. Instead of a hashfunction, there can also be used other suitable functions for formingthe derived value.

Alternatively or additionally, for forming the derived value there canbe used an authentication check value (message authentication code, MAC)which for example is additionally formed via the above-described hashvalue. An authentication check value usually is shorter than a hashvalue and can therefore be completely displayed also on a small displaydevice of a data carrier and thus without security loss. However, forcalculating an authentication check value one needs a secret datum whichis known only to the data carrier and to the terminal, for example a PINor the like.

According to an embodiment of the method of the invention there isprovided a PIN or an analogous secret datum known only to the terminaland to the data carrier but otherwise to be kept secret. Such a PIN canbe used, as mentioned, to form an authentication check value which inturn can be used to form the value derived from a public key. Such aPIN, however, can also be used to encrypt a public key before thetransmission, for example from the data carrier to the terminal. Withthis, the transmission of the key is already secured againstman-in-the-middle attacks. Also other steps of protocols used can besecured by means of the PIN, as described below, in particular settingup a secure communication channel itself

The PIN or an analogous secret datum can in this connection be generatedor specified by the data carrier or by the terminal. On the one hand, itis possible that the data carrier specifies the PIN, for example by itbeing stored in the data carrier or being generated within the datacarrier and being displayed on the display device of the data carrier.The displayed secret datum can then be input to the terminal via aninput device, for example, a keyboard. However, vice versa, also theterminal can generate and display the PIN, which is then input to thedata carrier via a corresponding input device or is in any other waytransmitted to the data carrier.

In general, there can also be provided a further method according to theinvention for setting up a secured communication channel between aportable data carrier and a terminal on the basis of a crypto systembased on a secret datum, i.e. the crypto system here needs notnecessarily be additionally based on a described asymmetric keyarchitecture. Normally, however, this will be the case. According tosuch a method, the secret datum is made available by the terminal, forexample is generated therein and displayed on a display device of theterminal. The displayed secret datum is subsequently transferred to thedata carrier. A corresponding data carrier according to the inventioncomprises a display device as well as a control device for setting up asecured communication channel to a terminal on the basis of a cryptosystem based on a secret datum via a data communication interface of thedata carrier. The control device here is adapted, according to theinvention, to display a secret datum received via the data communicationinterface or an input device of the data carrier on the display device.

The secret datum here can be transferred to the data carrier by a userof the data carrier inputting it to the data carrier via an input deviceof the data carrier, for example a keyboard. The input device here canalso be configured in an alternative manner, for example as an opticalsensor or the like. The input secret datum can then be displayed on thedisplay device of the data carrier. Such a display step, however, canalso be omitted.

It is also possible to transfer the secret datum to the data carrier,according to a suitable communication protocol, in a contactless orcontact-type manner, via an ordinary data communication interface of thedata carrier. The secret datum transferred in this way is then displayedon the display device of the data carrier for the purpose of verifyingthe error- and manipulation-free transfer.

DESCRIPTION OF THE DRAWINGS

In the following the invention is described by way of example withreference to the accompanying Figures.

FIG. 1 shows a preferred embodiment of a data carrier according to theinvention and

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 2 to FIG. 6 schematically show steps of different embodiments ofthe method according to the invention.

With reference to FIG. 1 a portable data carrier 10, which here isrepresented as a chip card, comprises a data communication interface 20in the form of a contact field adapted for the contact-type datacommunication and a data communication interface in the form of anantenna coil 22 adapted for the contactless data communication. One ofthe two data communication interfaces 20, 22 may be omitted. The energysupply of the data carrier 10 can be effected via the data communicationinterfaces 20, 22 through the terminal or via a separate energy supplyof the data carrier 10, for example a battery (not shown).

The two data communication interfaces 20, 22 are connected with anintegrated circuit 30 embedded in a data carrier body, whereby theintegrated circuit controls the data carrier 10 by means of a controldevice 32.

Furthermore, the data carrier 10 comprises a display device 40 in theform of a display screen as well as an input device 50 in the form of akeyboard. Alternative or further display and input devices can beprovided. The display device 40 is adapted to display contents generatedby the control device 32 and transmitted to the display device 40. Theinput device 50 can be used to input data to the data carrier 10, whichthen are further processed by the control device 32.

The control device 32 is in particular adapted to set up, on the basisof an asymmetric crypto system, a secured data communication channel toa terminal (not shown) via one of the data communication interfaces 20,22. The control device 32 is adapted to send a public key of the datacarrier 10 to a terminal or to receive a public key of a terminal and tofurther process it depending on the communication protocol used, forexample for determining a secret session key for symmetricallyencrypting data to be transmitted between the data carrier 10 and theterminal. The control device 32 can form values 42 derived from thepublic keys, for example by means of a hash function and/or anauthentication check value, and transmit them to the display device 40for displaying. Further, the control device 32 is adapted to generate asecret datum, for example in the form of a PIN or the like, and tooptionally transmit it to the display device 40 for displaying or toreceive such a secret datum via a data communication interface 20, 22 orvia the input device 50, to manage it and to further process it asintended.

Different methods for setting up a secured communication channel betweenthe data carrier 10 and a terminal are described in more detail in thefollowing with reference to the FIGS. 2 to 5.

With reference to FIG. 2, according to a first embodiment, in the steps100, 101 there are exchanged public keys between the data carrier 10 andthe terminal, i.e. data carrier 10 sends its public key PK_(D) to theterminal in step 101, which in step 100 sends its public key PK_(T) tothe data carrier 10.

In step 110, the terminal forms, by means of a hash function H, a valueW′=H(PK_(D)) derived from the received public key PK_(D) of the datacarrier 10 and displays this value W′ on a display device of theterminal, for example a display screen.

Analogously, also the data carrier 10 determines a value W=H(PK_(D)), byit applying the hash function H to its public key PK_(D). The datacarrier 10 also displays this value W via its display device 40 in step111.

A user of the data carrier can therefore check in step 120 whether thetwo values W and W′ match. If this is the case, the user can assume thatthe public key PK_(D) has not been manipulated by an unnoticedman-in-the-middle attack upon the transmission from the data carrier 10to the terminal.

In the same way, now the terminal determines a derived valueV′=H(PK_(T)) by applying the hash function H to its public key PK_(T)and displays this value in step 130 on its display device.

Likewise, in step 131 the data carrier 10 displays on its display device40 the derived value V=H(PK_(T)), which results from an application ofthe hash function H to the public key PK_(T) of the terminal receivedfrom the terminal.

The user checks in step 140 also the identity of these two values V andV′ and can be sure that, if identity is given, the public key PK_(T) ofthe terminal has been received unchanged in the data carrier 10.

If both checks 120, 140 turn out positive, the communication channelbetween the data carrier 10 and the terminal can be securely set up. Inthe opposite case, when at least one of the two checks 120, 140 hasyielded non-identity, the method is aborted 150.

Instead of a complete hash value H(PK_(T)) or H(PK_(D)), there can alsobe displayed in the steps 131 and 111 only a portion of a correspondinghash value, if the display device 40 of the data carrier 10 cannotrepresent the complete hash value. The comparison in the steps 120, 140then relates only to the corresponding displayed portion and accordinglyinvolves some uncertainty. Instead of a hash function H, there can beused a different, suitable function for forming the derived values V,V′, W, W′. It is also possible that the derived values are not derivedfrom the public keys PK_(T), PK_(D), but from values directly orindirectly derived from these keys, for example from a session keydetermined by means of the keys.

A variation of the method according to FIG. 2 is represented in FIG. 2a. According to the represented embodiment, values of public keysderived in the steps 1000, 1010 are exchanged between data carrier 10and terminal. Here, the terminal sends in step 1000 a value V₂=H(PK_(T))of its public key PK_(T), derived by means of a hash function, to thedata carrier 10, which in step 1010 sends a value W₂=H(PK_(D)) of itspublic key PK_(D), derived by means of a hash function, to the terminal.

The hash functions used therefor are preferably collision-resistant hashfunctions, such as for example SHA-2.

Subsequently, in step 1100 the terminal sends the public key PK_(T) tothe data carrier 10, which in step 1310 checks the derived valueV₂=H(PK_(T)). Accordingly, the data carrier 10 sends the public keyPK_(D) in step 1110 to the terminal, which in step 1300 checks thederived value W₂=H(PK_(D)).

Both the terminal and the data carrier now calculate in step 1400 or1410 a common secret CS′=PK_(D)^SK_(T) or CS=PK_(T)^SK_(D) according tothe asymmetric Diffie-Hellman method.

From the two values CS′ and CS, in step 1400 and 1410, there is furtherformed by means of a hash function a derived value, which, asrepresented in step 1500, 1510, is displayed respectively. Here, too,there can be displayed only a part of the hash function, when thedisplay device 40 of the data carrier 10 is not able to represent itcompletely. A different function than a hash function is alsoconceivable for forming the derived values. Moreover, the derived valuesmay not be determined directly from the public keys, but from valuesderived directly or indirectly from these keys.

The user checks the match of the two values in step 1600. If this is thecase, the user can assume that the public keys PK_(D) and PK_(T) werenot manipulated through a man-in-the-middle attack upon the transmissionfrom the data carrier to the terminal and vice versa. Accordingly,thereafter, a communication channel can be securely set up. Otherwise,as represented in step 1700, the connection is interrupted.

The method illustrated with reference to FIG. 3 differs from the methodin FIG. 2 by the fact that for forming the values X, X′, Y, Y′ derivedfrom the public keys PK_(T), PK_(D) exchanged in the steps 200, 201,besides a hash function H there is also formed an authentication checkvalue (MAC, message authentication code) on the basis of a PIN. Forthis, a user of the data carrier 10 inputs in step 205 a PIN to theterminal via an input device, e.g. a keyboard.

The PIN is specified in this case by the data carrier 10. In thesimplest case, the PIN can be firmly assigned to the data carrier 10upon the manufacturing, for example be printed on or stored in a memoryof the data carrier 10. The data carrier 10 can also be adapted togenerate the PIN session-specifically and to display it, as a storedPIN, on the display device 40 when required. It is also possible, butnot represented in FIG. 3, that the PIN is stored in the terminal or isgenerated session-specifically in the terminal and is displayed via thedisplay device of the terminal. There the user then can read the PIN andinput it to the data carrier via the input device 50. Other ways fortransfering the PIN from the terminal to the data carrier are possible.

In the steps 210, 211 and 230, 231 now the terminal or the data carrierdo not display on their respective display devices the hash valuesH(PK_(D)), H(PK_(T)) over the respective public keys PK_(D), PK_(T), butauthentication check values MAC_(PIN) (H(PK_(D))) or MAC_(PIN)(H(PK_(T))) which have been formed by means of the PIN in the known wayover the hash values H(PK_(D)), H(PK_(T)), for example by means of blockciphers. Such a MAC, formed over a hash value, is shorter than the hashvalue and can thus be completely displayed also on a small displaydevice of the portable data carrier 10, and thus without securitylosses. The MAC can alternatively be formed also directly over therespective public keys PK_(D), PK_(T).

Accordingly, in the steps 220 and 240 there is checked the identity ofthe MAC-values instead of the identity of the hash values.

The method represented with reference to FIG. 4 differs from the methodin FIG. 3 by the fact that the check of the identity of the MAC-valuesis not visually checked by the user, but by machine through the terminaland the data carrier 10. For this purpose, in step 312 the user inputsthe MAC-value X=MAC_(PIN)(H(PK_(D))), displayed in step 311 on thedisplay device 40 of the data carrier 10, to the terminal. There, instep 320 this value is compared with the MAC-valueX′=MAC_(PIN)(H(PK_(D))) already determined in step 310 within theterminal. If the two values do not match, the terminal aborts themethod.

If the values match, then, vice versa, the terminal displays, as in themethod in FIG. 3, the MAC-value Y′=MAC_(PIN)(H(PK_(T))) on its displaydevice in step 330. The user inputs the value Y′ in step 332 to the datacarrier 10 via the input device 50 of the data carrier 10. There, theinput value is compared in step 340 with the MAC-valueY=MAC_(PIN)(H(PK_(T))) determined within the data carrier in step 311.In case of non-identity, the data carrier 10 aborts the method,otherwise it is continued as provided.

As already described with reference to FIG. 3, also in this embodimentaccording to FIG. 4, the PIN can be alternatively specified by theterminal.

Likewise, it is possible to combine the concept represented withreference to FIG. 4 with the embodiment of FIG. 2. I.e., inputting thederived value displayed by means of one of the devices to the respectiveother device for the guaranteed faultless machine-check there, is inprinciple also possible without a PIN. In the method according to FIG. 2there are calculated only hash values, but no MACs, so that a PIN is notrequired.

With reference to FIG. 5 there is described an embodiment in which onlyone public key, namely that of the data carrier 10, PK_(D), istransferred to the terminal in step 401. In the steps 410 and 411 thereare again displayed, as already in the method of FIG. 2, a hash valueW′=H(PK_(D)) by the terminal, calculated over the key PK_(D) receivedfrom the data carrier 10, and a hash value W=H(PK_(D)) by the datacarrier 10, calculated over the public key PK_(D) assigned to the datacarrier 10. The user checks in step 420, as described, the identity ofthese two displayed values W, W′.

In a manner also already described, a PIN is input to the terminal instep 425. This step can also be omitted.

The terminal determines in step 430 in a suitable manner, possibly usingthe PIN, a secret key K which in step 440 is transmitted as valueN=ENC_(PK) _(D) (K, PIN), possibly together with the PIN, encrypted withthe help of the received public key PK_(D) to the data carrier 10.

The data carrier 10 decrypts the value N by means of its private keySK_(D) of the crypto system, in order to get the secret key K, andoptionally checks the PIN also received with the message N.

The method according to FIG. 5 can be modified to the effect that thepublic key PK_(D) is already encrypted by means of the PIN before it istransmitted to the terminal. The PIN here can be input to the terminalalready before the transmission of the key PK_(D). At least, inputtingthe PIN must be effected before the display of the hash values, becauseotherwise the hash value over the unencrypted key PK_(D) cannot beformed within the terminal.

In a further, not shown method for setting up a secure communicationchannel, the terminal displays in a first step, as described above withreference to FIG. 3, a secret datum in the form of a PIN (personalidentification number), of a SIN (session identification number) or thelike. The secret datum is subsequently input to the data carrier 10 viathe input device 50 by the user. Alternatively, it is possible totransmit the secret datum to the data carrier via a different way, forexample via an optical sensor, via an air interface (OTA) or the like.For verifying purposes, the transferred secret datum can be displayed onthe display device 40 of the data carrier 10, in particular when it hasnot been input by the user itself Now, every method based on such asecret datum, which does not necessarily depend [on—added by theTranslator] an asymmetric crypto system, can be used to set up a securedcommunication channel—at least partly based on the secret datum.Normally, in such methods, too, an asymmetric crypto system is involved,as for example described with reference to FIGS. 3 to 5. Also knownmethods based on a secret datum, such as PACE, PACE-EU, SPEKE and thelike, which do not require a display device in the data carrier to setup a secure channel between the data carrier and the terminal, can becarried out in the described manner.

Instead of an alphanumeric secret datum, there can also be provided thetransmission of a picture or of a pictogram. In this way, to a user ofa—contactless—data carrier 10 it can be represented for example easierwith which terminal the data carrier 10 is connected.

A simple data carrier without a display device and without an inputdevice can be extended for example by means of a mobile radio enddevice, with which the data carrier, e.g. a (U)SIM mobile radio card, isconnectable, with the necessary functionalities for carrying out thedescribed methods. I.e. the display device and the input device are madeavailable through the mobile radio end device connected with the datacarrier.

It is further possible to set up a secure data communication channelbetween two contactlessly communicating data carriers 10 in the waydescribed, i.e. one of the data carriers plays the role of the terminalhere.

In a further variant of the invention there is first formed, asrepresented in FIG. 6, in step 500 a random number ZZ1 by the datacarrier 10, therefrom is formed a derived value H(ZZ1) for example bymeans of a hash function, and the value H(ZZ1) and a public key of thedata carrier PK_(D) are sent to the terminal. Subsequently, the terminalencrypts a second random number ZZ2 with the public key PK_(D) in a step510 and sends the such obtained value B to the data carrier 10.

The data carrier 10 determines the value of ZZ2 by the decryption of Bwith the help of the private key SK_(D) of the asymmetric key pairPK_(D)−SK_(D) in step 520.

The data carrier 10 sends in step 530 the random number ZZ1 to theterminal, which forms in step 540 the derived value H(ZZ1) and comparesit with the value H(ZZ1) transmitted by the data carrier in step 500.

In a further step 550 and 551, there are formed both by the terminal andby the data carrier 10 the derived values D′ and D as derived valuesfrom ZZ2, ZZ1 and PK_(D), and are displayed in step 560 and 561. Asdescribed in the preceding implementation variants, the values of D andD′ are here, too, compared with each other by the user and acommunication channel is set up only after a positive match. Otherwise,the communication between data carrier and terminal is aborted.

The random number ZZ2, which the terminal specifies, can be a so-calledsession key, which in the course of the further communication is usedbetween terminal and data carrier.

Preferably, the user makes a declaration of will before the setting upof a communication connection to the terminal. The input of one orseveral characters to the input device of the data carrier or any othersuitable declaration vis-à-vis the data carrier, such as bending thecard, pressing a key or the like, makes it unmistakably clear that theuser is willfully using the data carrier. Making the declaration of willcan be effected at any time of the method according to the invention andof the described variants according to FIGS. 2 to 6, but preferably atthe beginning or at the end of the method, but in any case before thesetting up of the secure communication channel.

The invention claimed is:
 1. A method for setting up a securedcommunication channel between a portable data carrier and a terminal onthe basis of an asymmetric crypto system, the method comprising:deriving a value from a public key of the asymmetric crypto system; anddisplaying the value on a display device of the portable data carrier,wherein the portable data carrier is a chip card.
 2. The methodaccording to claim 1, including displaying on a display device of theterminal a value derived from a public key of the crypto system.
 3. Themethod according to claim 1, including displaying both on the displaydevice of the portable data carrier and on the display device of theterminal both a value derived from a public key assigned to the datacarrier and a value derived from a public key assigned to the terminal.4. The method according to claim 1, wherein the value from a public keyand displayed on the display device of the terminal is input to theportable data carrier via an input device of the portable data carrierand is compared in the data carrier with a value which has been derivedfrom a public key received by the data carrier.
 5. The method accordingto claim 1, wherein the value derived from a public key and displayed onthe display device of the data carrier is input to the terminal via aninput device of the terminal and is compared in the terminal with avalue which has been derived from a public key received by the terminal.6. The method according to claim 1, wherein the value derived from apublic key of the crypto system is transmitted to a communicationpartner before the associated public key.
 7. The method according toclaim 1, wherein the value derived from a public key of the cryptosystem is formed by means of a hash function.
 8. The method according toclaim 1, including generating a common secret on the basis of anasymmetric crypto system using the public and the private keys of theterminal and of the data carrier.
 9. The method according to claim 1,wherein the value from a public key of the crypto system is formed bymeans of an authentication check value.
 10. The method according toclaim 9, wherein a PIN for forming the authentication check value ismade available.
 11. The method according to claim 10, wherein the PIN isstored or is generated in the terminal, is displayed on a display deviceof the terminal and is input to the data carrier via an input device ofthe data carrier.
 12. The method according to claim 1, including makinga PIN for the encrypted transmission of the public key of the datacarrier to the terminal.
 13. The method according to claim 12, whereinthe PIN is specified by the data carrier and is input to the terminalvia an input device of the terminal.
 14. The method according to claim13, wherein the PIN is generated in the data carrier and is displayed onthe display device of the data carrier.
 15. The method according toclaim 1, wherein before the setting up of a secured communicationchannel, the user of the data carrier makes a declaration of willvis-a-vis the data carrier.
 16. The method according to claim 15,wherein the declaration of will is effected with the help of the inputdevice of the data carrier.
 17. A method for setting up a securedcommunication channel between a portable data carrier and a terminal onthe basis of a crypto system based on a secret datum, the methodcomprising: making available the secret datum by the terminal,displaying the secret datum on a display device of the terminal, andtransferring the secret datum to the data carrier, wherein the portabledata carrier is a chip card.
 18. The method according to claim 17,including transferring the secret datum to the data carrier by inputtingthe secret datum to the data carrier via an input device of the datacarrier.
 19. The method according to claim 17, wherein the secret datumis displayed on a display device of the data carrier.
 20. The methodaccording to claim 17, wherein the method is further based on anasymmetric crypto system and comprises deriving a value from a publickey of the asymmetric crypto system; and displaying the value on adisplay device of the portable data carrier.
 21. A portable data carriercomprising: a display device, and a control device configured to set upa secured communication channel to a terminal on the basis of at leastone of an asymmetric crypto system and a crypto system based on a secretdatum via a data communication interface of the data carrier, whereinthe control device is arranged to display a value derived from a publickey of the asymmetric crypto system on the display device and/or todisplay a secret datum received from the terminal via the datacommunication interface or an input device of the data carrier on thedisplay device, wherein the portable data carrier is a chip card. 22.The data carrier according to claim 21, wherein the data carrier and thecontrol device are arranged to derive a value from a public key of theasymmetric crypto system, and display the value on a display device ofthe portable data carrier.
 23. A system comprising a data carrieraccording to claim 21, and as a terminal, wherein the terminal and thedata carrier are configured to derive a value from a public key of theasymmetric crypto system, and display the value on a display device ofthe portable data carrier.